1. Field of the Invention
The present invention relates generally to a valve train for an internal combustion engine or the like and more specifically to a variable valve timing arrangement therefor.
2. Description of the Prior Art
In a known arrangement such as shown in FIG. 1 of the present application, it has been proposed to operate a poppet valve, such as an inlet or exhaust valve of an internal combustion engine, via a rocker arm 1 which engages a cam 2 at one end and which is pivotally mounted on top of the stem 3 of the valve 4 at the other end. The upper surface of the rocker arm 1 is contoured and adapted to abut a lever 5. The point of abutment with the lever 5 defines the pivot or fulcrum point of the rocker arm. With this arrangement as the cam 2 rotates the rocker arm 1 is cammed to pivot about the fulcrum point defined by the aforementioned contact and induce the valve 4 to reciprocate. To vary the timing and degree of lift the valve 4, a second cam 6 is provided and adapted to abut the lever 5. The second cam 6 is selectively rotated by a suitable hydraulic motor or the like (not shown). Thus, if the second cam 6 is rotated in a direction to urge the lever 5 to rotate counter-clockwise (viz., downwardly as seen in the drawings) the degree of valve lift and the duration for which the valve is open will be increased. Rotation of the cam which allows the lever to point in the clockwise direction (as seen in the drawings) reduces the valve lift and the duration for which the valve is open.
However, this arrangement has suffered from the drawbacks that the provision of the cam and lever arrangement above the rocker arm increases the overall height of the engine and, as the lever/cam arrangement does not permit ready adjustment of the clearance between the rocker arm and the top of the valve stem, a rather large clearance must be provided to allow for thermal expansion, wear etc. This clearance unavoidably leads to the generation of so called "tappet noise", vibration and also tends to deteriorate the valve timing itself. A further drawback comes in that, in the case the above disclosed arrangement is applied to an engine having four or more cylinders, as the cams are usually disposed on the same common cam shaft for the purpose of simplicity, the shaft is constantly subjected to reaction forces produced by the valve springs acting thereon through the rocker arms and levers which forces tends to rotate the shaft back against the bias applied by the servo. These forces tend to peak during engine operation as each valve lift reaches its zenith and the fulcrum point defined between each lever and rocker arm moves in the direction of the cam. Thus, in the case wherein a single servo is connected to one end of this cam shaft, it must be able to produce sufficient power to both maintain the shaft in any given desired position against this reaction force as well as overcoming the friction generated between the bearings etc., of the shaft by the reaction force when it is desired to vary the valve timing.
This latter drawback is particularly manifest in four cylinder engines wherein a valve is always being lifted.
One method for overcomming this problem would be to provide a servo and cam shaft for each valve, however this would lead to a prohibitively complex arrangement which would be both heavy and difficult to precisely control. For a complete disclosure of the above described arrangement reference is made to U.S. Pat. No. 3,413,965 issued on Dec. 3, 1968 in the name of J. M. Gavasso.